1. Field of the Invention
This invention relates to lens systems and more particularly to a novel lens system of image formation and photographic recording which insures that an image will be in focus on the image plane of a still, TV or movie camera.
2. Description of the Prior Art
Conventionally, in most optical systems for image formation and photographic recording such as in still photography and movie recording, the image plane in the system is in a fixed relationship with respect to a data plane on the camera body. It has been the conventional practice to establish the data plane as the lens mounting face on the front of the camera. The camera manufacturer, in order to provide the fixed relationship, provides various lenses designed which when used with their particular camera, assures that an image will be in focus on the image plane.
In some instances, attempts have been made to move the image plane with respect to the data plane and in such systems, the image plane is adjustable in terms of focus as well as focus adjustments on the lens used with the systems. This latter instance is employed mainly with color television cameras. In such cameras, three individual pick-up tubes are used so that three independently adjustable image planes are employed to compensate for any variance through a prism splitting system as well as variance in pick-up tube tolerance and dimensions. Also, television systems employ zoom lenses which, if not properly set, will not track focus throughout their full zoom range. For example, if the back focal distance between zoom lens and image plane is incorrectly set somewhere within the zoom range, the image will loose sharp focus.
In actual practice, the usual procedure for establishing a zoom lens camera combination is to point the camera with the lens attached at a distant object, at or near infinity, with the lens zoomed to the widest field condition. At this point, the lens to image plane distance is varied to achieve best focus. This procedure is accomplished with the lens at its wide open aperature so depth of focus is shallowest. The lens is then zoomed to narrowest angle and the lens focus adjusted for best focus. After moving the lens back and forth between wide and narrow zoom conditions two or three times, the lens camera is considered adjusted or best focused. From the foregoing, it can be recognized that two important conditions are impossible to achieve when employing the camera indoors. For example, there is no condition of infinity or distance objects available with which to achieve focus, and exteriors with high levels of illumination having the aperature wide open is impossible to reach except with neutral density filters.
Therefore, with the advent of electronic journalism, additional needs have been required for cameramen in the field to be able to change lenses and be able to quickly and accurately check camera lens back focal length relationships.